Gas sensor devices are utilized in a number of sensing applications. Dual-cell gas sensors, for example, are frequently used to measure oxygen, which is particularly important in automobile and engine systems. One type of dual-cell gas sensor measures the concentration of a gas component in a first space comprising a sealed measurement space, of which at least one wall portion consists of a separation wall which exhibits ionic conduction and is in contact at least in part via the outer side with the first space. In this type of dual-cell gas sensor, a control unit can be utilized to periodically supply during a pumping time interval, a pumping current to the separation wall so that by means of an ion current in the separation wall the gas component is removed from the measurement space.
In this type of device, during a filling time interval a filling current can be supplied whose polarity is opposite to that of the pumping current so that the gas component is supplied to the measurement space. The dual-cell sensor includes a detection circuit which is connected to electrode layers on either side of the separation wall, the outer electrode layer of which is in contact with the first space. This detection circuit includes a first voltage detector, which supplies a filling interrupt signal for interrupting the filling current when the electrode voltage across the said electrode layers reaches a first reference value, and a second voltage detector which supplies a pumping interrupt signal for interrupting the pumping current when the electrode voltage reaches a second reference value. The electrical charge provided in the separation wall is a measure of the concentration of the gas component. Such a gas analysis apparatus is disclosed in U.S. Pat. No. 4,384,935 entitled “Gas Analysis Apparatus” which is incorporated herein by reference.
During the measurement of the electrical charge provided in the separation wall it is assumed that the separation wall, as to its impedance, acts as an electrical resistance so that this charge is to be measured outside the separation wall as supplied and removed charge or as a product of a current to be measured and a time interval to be measured or with constant currents as time intervals.
However, when the various parameters, such as the temperature, the volume of the sealed measurement space, the chosen measurement currents and the measuring range of the concentration to be measured, have such values that the measured time intervals become comparatively small, it is found that the measurement is strongly influenced by switch-on and switch-off transients. In other words, the separation wall is not a pure resistance. It can be derived from a theoretical consideration that the equivalent circuit diagram of the separation wall comprises besides resistances also capacitances, as a result of which RC time constants and stored capacitor charges are obtained.
Another type of dual-cell sensing device is disclosed in U.S. Pat. No. 4,545,889 entitled “Gas Analysis Apparatus” which describes a gas analysis apparatus for measuring the concentration of a gas component in a first space. The apparatus described in U.S. Pat. No. 4,545,889 includes a sealed measurement space, of which at least one wall portion consists of a separation wall which exhibits ionic conduction. The concentration of the gas component in the measurement space is changed periodically between two values by filling and pumping currents at the separation wall. The time intervals are measured and are a measure of the concentration. However, these time intervals comprise a “dead time” caused by switch-on and switch-off (both electrical and physical) transients. When given time intervals are combined by addition and subtraction, the influence of dead times can be considerably reduced using the device of U.S. Pat. No. 4,545,889.
Such dual-cell type sensor devices, however, are plagued with a number of problems. First, a typical dual cell pellet type configuration is designed for flue gas environment and must be redesigned for automotive applications. Thus, the use of dual-cell type sensing devices in automotive applications is very limited. The cost of such sensors is also extremely high, particularly in the context of automotive engine control applications. Dual cell sensors are also bulky and offer a slow response, which means that such devices need to be improved considerably for engine control applications. Additionally, these type of devices are fragile and utilize glass and/or ceramic seals, which are also delicate components, which means that the devices must be re-designed for automotive applications.
It is therefore believed that a solution to these problems lies in eliminating the dual-cell nature of such devices and completely re-designing a much simpler and efficient device, one which is based on the use of a single cell sensing element for oxygen sensing applications. Such a device and operating method are described in greater detail herein.